It is not uncommon for undetected bleeding to occur during surgical procedures because of an unintentionally severed vein or artery. The ensuing loss of blood can result in serious deterioration in the patient's condition, or even in death, if it is not promptly stemmed.
Blood pressure and pulse rate are monitored by the anethesiologist during a surgical procedure, and these parameters provide valuable information on the patient's condition. However, because systemic vascular resistance can increase dramatically during episodes of blood loss such as those just described, it may be as long as four or five minutes after serious bleeding develops or a blood vessel is severed before an appreciable decrease in pulse rate or blood pressure occurs. Currently, serious undetected losses of blood can occur in periods of this magnitude because as much as 35 percent of a patient's blood may be lost before there is a noticeable decrease in blood pressure. By then, however, the patient may be going into shock or suffering other complications attributable to the loss of blood. Although blood pressure and pulse may remain relatively constant for this extended period of time, cardiac output begins to decrease coincidentally with the loss of blood. Hence, under these conditions by monitoring this parameter, loss of blood can be detected much earlier than would otherwise be the case. This permits the surgical team to take prompt remedial action, hopefully forestalling the deterioration in the patient's condition that might have occurred had the loss of blood gone unchecked.
Variations in cardiac output can also be utilized to detect other unwanted changes in the patient's condition such as the onset of schemia of the heart muscle or an anesthetic reaction, again permitting remedial action to be taken before there is any significant deterioration in the patient's condition.
Thermal dilution is one technique which has heretofore been employed to measure cardiac output. In that technique, a thermal dilution catheter carrying a thermistor on its tip is inserted through an incision into the jugular vein and threaded through that vessel and the right side of the patient's heart into the pulmonary artery. A saline solution is then injected through the catheter into the patient's bloodstream, typically at a temperature of 0.degree. C. This solution mixes with the blood flowing through the pulmonary artery, monentarily reducing the temperature detected by the catheter tip thermistor. Standard thermodynamic equations allow the cardiac output to be determined from this drop in temperature and the volume of saline solution which produced the temperature drop.
The thermal dilution technique of measuring cardiac output has the disadvantage that is is highly invasive and therefore potentially capable of damaging the anatomical structures through which the catheter is threaded. In fact, in a small percentage of cases (one to two percent), serious complications result from employment of the thermal dilution technique.
Also, the mere presence of the catheter in the pulmonary artery may result in localized clotting of the blood flowing through that vessel. This can obstruct the orifice through which the saline solution is discharged or produce an insulating layer around the thermistor. In both cases, the results will be highly inaccurate.
Finally, the thermal dilution technique is time consuming as it may take as long as 30 minutes to place the catheter; and only a limited number of measurements per hour of cardiac output can be made. Changes in a patient's condition requiring prompt remedial action may therefore not be detectable by the thermal dilution technique.
Because of the disavantages discussed above, the thermal dilution technique for measuring cardiac output is generally employed only if the patient is undergoing cardiac surgery or is sufficiently ill that surgery poses a risk of cardiac failure.
The Fick method is another technique for measuring cardiac output that has heretofore been employed to some extent. In it, blood samples are taken at two different points in the circulatory system, one just downstream of the patient's aorta and the other in the pulmonary artery. The concentrations of oxygen in these arteries are compared and combined with the amount of carbon dioxide being expelled by the patient to provide a measurement of cardiac output.
The Fick technique has the disadvantage that the measurements are complex and can easily require a day of analysis before cardiac output can be ascertained. This makes the Fick technique useless in the operating theatre where up-to-the-minute information is required to keep the patient in a stable condition.
Of the techniques for measuring cardiac output discussed above, thermal dilution is the most widely employed.
The drawbacks and disadvantages of the above-discussed techniques for measuring cardiac output are eliminated in the method of measuring cardiac output described in U.S. Pat. No. 4,509,526 issued Apr. 9, 1985, to Barnes et al. for METHOD AND SYSTEM FOR NON-INVASIVE ULTRASOUND DOPPLER CARDIAC OUTPUT MEASUREMENT. U.S. Pat. No. 4,509,526 is assigned to the assignee of this application and is hereby incorporated herein by reference.
In the method of measuring a patient's cardiac output disclosed in the Barnes et al. patent, the diameter of the patient's ascending aorta is determined by a pulsed-echo transducer placed on the chest and the systolic velocity of the blood flowing through that artery is determined by insonification of the aorta with an ultrasonic suprasternal notch probe. This second probe makes available Doppler or frequency-shifted electromagnetic signals which are analyzed and converted from the time domain into discrete frequency components by digital fast Fourier transform. The Doppler shifted frequency components of the return signal are converted to velocities, and the latter are employed to calculate a systolic velocity integral.
Multiplying the systolic velocity integral by the cross-sectional aortic area yields beat-by-beat cardiac stroke volumes of the patient; summing the stroke volumes over a predetermined number of consecutive beats and then dividing by the time spanning the predetermined number of beats (in other words, multiplying by the heart rate), yields the patient's cardiac output.
The patented cardiac monitoring apparatus facilitates direct operator interaction with the apparatus over the course of the measurement protocol via a touch sensitive visual display which, inter alia: instructs the operator at each step of the sequence and responds to the election of operator options with failsafe features that guard against the entry of invalid data and otherwise minimize operator error. The operator may interact without extensive training, and the system provides the benefits of microprocessor control including fast data processing without elaborate hardware or software.
Within operational limits, the patented cardiac monitoring system will insist upon the entry of required data, will limit the entry of certain data to values within statistically anticipated ranges, and will assist the operator in optimizing the measurement of variable parameters.
In the novel method and apparatus for monitoring cardiac output we have invented, an ultrasonic esophageal probe is substituted for the suprasternal notch probe used to monitor systolic velocity in the system disclosed in U.S. Pat. No. 4,509,526. This probe monitors the blood flowing through the patient's descending aorta rather than his ascending aorta. This velocity is integrated, and the result is combined with a number representing the area of the patient's ascending aorta to produce a cardiac output value.
This substitution of an ultrasonic esophageal probe for the suprasternal notch probe employed in the patented equipment is important when the system is used during surgery. The preferred type of esophageal probe can be held in position for an extended period of time as may be necessary during major surgery, for example. Furthermore, it does not interfere with the operating field as does a suprasternal notch probe of the type disclosed in U.S. Pat. No. 4,526,509. In addition, unless esophageal surgery is involved, the probe is out of the sterile field, which is an obvious advantage. Furthermore, this probe replaces the esophageal stethoscope which would be employed in any event so that, in effect, another measurement of the patient's condition can be monitored without further invasion of the patient's body.
The blood flowing through a patient's descending aorta is only about 70 percent of that flowing through his ascending aorta, the remainder having been distributed to the patient's subclavian and carotid arteries before the descending aorta is reached. Consequently, in our novel cardiac output measuring apparatus, provision is made for scaling the systolic velocity measured by the esophageal probe by an appropriate conversion factor to the velocity which would have been obtained if the flow in the patient's ascending aorta were instead monitored before the integration is performed.
The proportioning of the blood pumped by a patient's heart between the descending aorta and those other blood vessels discussed above will vary from patient-to-patient. Consequently, we preferably use the suprasternal notch probe technique of measuring systolic velocity disclosed in U.S. Pat. No. 4,526,509 to determine an accurate conversion factor for each patient.
The technique for providing the aortic area value that we employ is also completely different from that disclosed in U.S. Pat. No. 4,526,509. In the patented technique, aortic diameter is measured by insonification of the patient's ascending aorta and converted to aortic area. We, instead, employ a predictively determined value of aortic diameter in our method of and apparatus for determining cardiac output. This has the advantage that it makes the cardiac monitoring equipment we have invented much simpler to use, lighter, and less expensive than that disclosed in U.S. Pat. No. 4,526,509.
The preferred method of predictively determining aortic diameter involves the solution of the following algorithm: EQU AD=12.6+[AGE+[C.sub.1 ]-[SEX.times.C.sub.2 +[HEIGHT.times.C.sub.3 ]+[WEIGHT.times.C.sub.4 ]
where:
AD is aortic diameter in inches, PA1 AGE is the age of the patient in years, PA1 C.sub.1 is in the range of 0.046 to 0.066, PA1 SEX is the sex of the patient, PA1 C.sub.2 is in the range of 0.7 to 1.3 if the patient is a female and in the range of 1.4 to 2.6 if the patient is a male, PA1 HEIGHT is the height of the patient in inches, PA1 C.sub.3 is in the range of 0.012 to 0.022, PA1 WEIGHT is the weight of the patient in pounds, and PA1 C.sub.4 is in the range of 0.09 to 0.17. PA1 C.sub.1 : 0.066, PA1 C.sub.2 : 1.0 if the patient is a male and 2.0 if the patient is a female, PA1 C.sub.3 : 0.17, and PA1 C.sub.4 : 0.013. PA1 AD is aortic diameter in inches, PA1 AGE is the age of the patient in years, PA1 C.sub.1 is in the range of 0.046 to 0.066, PA1 SEX is the sex of the patient, PA1 C.sub.2 is in the range of 0.7 to 1.3 if the patient is a female and in the range of 1.4 to 2.6 if the patient is a male, PA1 HEIGHT is the height of the patient in inches, PA1 C.sub.3 is in the range of 0.012 to 0.022, PA1 WEIGHT is the weight of the patient in pounds, and PA1 C.sub.1 is in the range of 0.09 to 0.17; PA1 AD is aortic diameter in inches, PA1 AGE is the age of the patient in years, PA1 C.sub.1 is in the range of 0.046 to 0.066, PA1 SEX is the sex of the patient, PA1 C.sub.2 is in the range of 0.7 to 1.3 if the patient is a female and in the range of 1.4 to 2.6 if the patient is a male, PA1 HEIGHT is the height of the patient in inches, PA1 C.sub.3 is in the range of 0.012 to 0.022, PA1 WEIGHT is the weight of the patient in pounds, and PA1 C.sub.4 is in the range of 0.09 to 0.17;
Preferred values of the constants in the foregoing algorithm are:
In our novel cardiac monitoring apparatus this equation is solved automatically upon entry of the patient's age, sex, height, and weight.
Like that disclosed in U.S. Pat. No. 4,509,526, the justdescribed method for determining cardiac output disclosed herein is noninvasive and therefore does not subject the patient to the risk of infection or anatomical damage or require surgery as is the case in those cardiac output measuring techniques employing a catheter. And, as in the case of the patented method, that disclosed herein permits cardiac output to be monitored on a continuous, up-to-the-present moment basis.
Yet another advantage of our novel cardiac output monitoring apparatus is that it is capable of, or can easily be programmed to, furnish other valuable information regarding the patient. This includes stroke volume, cardia index, and systemic vascular resistance. Stroke volume was defined above. Cardiac index is cardiac output normalized by dividing that measurement by the patient's body surface area. In essence, this makes the cardiac output measurement patient independent. The person wanting the information can instead simply say that if a patient's cardiac index is above a specified level he is probably doing well and if his cardiac index is below that level there may be a problem. Systemic vascular resistance is blood pressure divided by cardiac output. This parameter can be employed to particular advantage in managing the administration of drugs.
Still other currently and potentially useful measurements that our novel cardiac monitoring apparatus is capable of providing are: peak systolic velocity, acceleration to peak, and delay in onset of systole.